Heat seeker with proportional control



Sept. 8, 1959 A. NYMAN ErAL 2,903,204

- HEAT SEEKER WITH PRoPoRTIoNAL CONTROL Train/fle- Sept. 8, 1959 I A.NYMAN ETAL HEAT SEEKER WITH PRoPoRTToNAL CONTROL 3 Sheets-Sheet 2 FiledNOV. 8. 1946 INVENTORS. FV El /Vll BY MEX/Wage #www Sept- 8, l959 A.NYMAN ETAL 2,903,204

HEAT SEEKER WITH PRoPoRTToNAL coNTRoL Filed Nov. 8, 1946 3 Sheets-Sheet3 United States Paten-t HEAT SEEKER WITH PROPORTIONAL CONTROL AlexanderNyman, Dover, Mass., and Fay E. Null, Dayton, Ohio Application November8, 1946, Serial No. 708,562

10 Claims. (Cl. 244-14) (Granted under Title 35, U.S. Code (1952), sec.266) The invention described herein may be manufactured and used by orfor the Government for governmental purposes without payment to us ofany royalty thereon.

This invention relates to heat seekers and more particularly to a deviceused to direct guide missiles to military targets which are giving outheat, or to a similar use.

In the past, devices of the type here considered were characterized byserious limitations in attempting:

To distinguish target radiation from background radiation at longdistances, as for example, ve miles or more;

To provide a quickly responding system, and

To provide control of the system in a degree that is proportional to thedeviation of the received signal from a predetermined direction asdistinguished from the all or none type of control in which every signalcauses the same control surface movement.

There are at present two kinds of systems commonly employed.

A continuous observation system in which heat sensitive elements orbolometers are arranged in multiple in a circular bank divided intoquadrants. Each quadrant is provided with its own path for transmittinga received signal to the control of a missile and the control isactuated by the signal to direct the missile toward the target,depending upon from which particular quadrant the strongest signalcomes.

A system in which the eld of view is scanned by a -to-and-fro orcircular motion so that successive portions of the field of View fallupon the detecting element in sequence. Directional sense is obtained bycommutation of the detector output as the scanning spot passes from onequadrant of the eld of View to the next quadrant.

The present invention belongs to the first class of such systems. Theinvention employs a plurality of radially disposed bolometers arrangedin each quadrant of a preferably circular bank and may employ as manycontrol circuits as there are bolometers.

One object of this invention is to provide an improved heat seekerarrangement by means of which the response of the heat detector willactuate the control system of a guided missile upon which it is mounted,with regard to two primary facts: (l) the position of the target withreference to the optical axis of the target seeker (i.e., whether thetarget is up or down, and right, or left of this axis). This fact willdetermine which control of the guided missile is to be actuated from amissile-carrying airplane and in which sense, in order to direct themissile to the target, and (2) the angular deviation of a line from thetarget seeker to the target with reference to the optical axis of thetarget seeker. The latter fact is used to determine the amount or rate-of control to be applied to the guided missile to restore it mosteffectively to a desired flight path. Flight path control over themissile is best accomplished by proportional control which has certainWell known advantages over the unproportional or ilicker control whichalters the flight path of the missile by a xed amount.

Advantages of the improved seeker-Proportional control The improvedseeker has the advantage of proportional control, whereby each bolometerin the receiver image area gives rise to a signal having its individualfrequency, thereby providing an output signal, the magnitude of which isproportional to the distance of the bolometer from the center of thebolometer bank or image area, and therefore proportional to the distanceof the target from the center of the field of view. Proportional controlis the most rapid and efficient type of control, minimizing the tendencyof a missile to overshoot or to hunt about the course to its target. Aproportional control system is substantially necessary for the controlof jet or rocket propelled missiles where an extremely fast controlsystem must be used.

A further advantage of the present invention is an automatic volumecontrol of an amplifier in the circuit of the seeker. It selects onlythe signal frequency from the hottest bolometer in the bolometer bank orimage area. Since a hot target radiates much more heat energy than itsbackground, where heat radiations of different intensities are appliedto two bolometers, signal from the hotter bolometer dominates and theseeker follows the target in the field of view since that targetradiates the more effective heat energy.

Another advantage of the improved seeker that is described herein ascompared with the usual scanning type of seeker is that energy is storedon a bolometer receiver as long as the position of the target does notchange with respect to the seeker axis. For example, in a scanningdevice covering image receivers per sec., energy only falls upon eachreceiver bolometer for 1/100 of a second. In the improved seeker, energywould be absorbed on a given receiver bolometer as long as the targetdid not move appreciably relative to the seeker axis. If the targetimage remained on one receiving bolometer for 0.5 second the amount ofenergy absorbed by the receiver would be 50 times the energy absorbed bythe receiver of 0.01 second scanning time per bolometer-f This extrasensitivity would be of special advantage at long ranges, when thereceived signal is very weak and the target would stay in approximatelythe same position relative to the seeker axis for a considerablefraction of a second.

A further advantage is that microphonic effects can be reduced to aminimum in the new seeker, since it contains no vibrating, rotating oroscillating mechanical parts such as mirrors, or optical heads.

Referring now to the accompanying drawings:

Fig. 1 is a block diagram of the electrical circuit for the presentseeker;

Fig. 2 is a functional diagram of the seeker, said diagram being partlyin block form and partly schematic, showing wave diagrams, shapes of thecurrents as they flow through particular pieces of apparatus making upthe assembly shown in Fig. 2. Arrows indicate which diagram correspondsto the current generated in a particular piece of apparatus;

Fig. 3 is a combination block and schematic circuit diagram (or of amodified circuit) further illustrating the combination of apparatusrequired to bring about the circuit functions. It corresponds in generalto Fig. 2, but different parts of that circuit are illustrated ingreater detail in Fig. 3 than in Fig. 2;

Fig. 4 is a front elevation of a circular bank of bolometers hereinaftercalled the bolometer-receiver;

Fig. 5 is a diagrammatic side elevation of a section of thebolometer-receiver and its associated optical system and electricalconnection;

Fig. 6 is an enlarged sectional view of a bolometer and its shieldingassembly and a schematic diagram of its associated bridge circuit;

Fig. 7 is a longitudinal section of a special variable resistor employedin the circuit;

Fig. 8 is a schematic diagram of that part of the circuit beyond thebolometer bridge outputs and shows connections enabling the use of acommon automatic volume control to the separate bolometer circuits andthe elimination of a mixer for said circuits. Associated wave diagramsconnected by arrows to the main diagram show the wave shapes at variouspoints of the circuit;

Fig. 9 is an alternative circuit to Fig. 8 which does not howevereliminate the automatic volume control. Associated wave diagrams areused as in Fig. 8.

Referring again to Fig. 4, 10 is a bolometer-receiver consisting of fourquadrants 10a, 10b, 10c, and 10d which are arranged in clockwise order.The receiver could be divided into more divisions if it were so desiredso long as they were symmetrically disposed. Each quadrant is dividedinto four bolometers 11a, 11b, 11c, and 11a', respectively, fromperiphery to center.

As will be seen from Fig. 6, the bolometers, for example, 11a, ismounted adjacent to a metal block 12 which bears another bolometer 13 onthe other side thereof. The reason for such construction is that theshielded bolometer 13 is provided merely for reference. That is to say,the Unshielded ones 11a, 11b, etc., are the only ones to be heateddirectly by the received signal. Therefore, a temporary heatdifferential is established between bolometer 11a and bolometer 13 longenough for the signal to actuate the circuits. After this isaccomplished, the signals heat flows into the block 12 and ultimatelybecomes distributed between the exposed bolometers 11a, etc., the block12 and the shielded bolometers 13, so that no heat difference remains tocause any electrical consequence.

The bolometers are not, however, mounted directly on the block 12. Aspace 14 remains under the central areas of the bolometers and thenupper and lower edges are gripped in mountings 15 of low electricalresistance. The bolometers themselves are thin deposits of metal mountedon a thin insulating film, for example of synthetic resin varnish.

The following method may be used to prepare the bolometers: A plasticplate may be molded with an opening the approximate size of thebolometer elements. The mounts 15 can be the edges of such a plate.After immersing such a plate in water, a thin plastic film can be formedon the surface of the water by pouring synthetic resin varnish thereonto the desired thickness. The plate can then be lifted out of water thefilm covering the opening. After the film dries, a mica stencil (notshown) is cut with openings the approximate size of the film and metalvapor is allowed to evaporate in vacuo and to deposit on the plasticfilm, thus forming the bolometer. Heavier metal deposits are then madeat the edges of which fine wires 16 are attached by means of Woodsmetal. Cellulose acetate or nitrate with a very small amount of Glyptallacquer has been found satisfactory for making the plastic film.

Referring now to Figs. 2 and 3, a master oscillator 17 is provided tocontrol a multi-vibrator 18, the latter being adapted to furnish sixteenharmonics of the fundamental frequency. Sixteen frequencies arenecessary because there are sixteen bolometer elements and each suchelement must be provided with its own frequency channel. The harmonicfrequencies are kept in the radio or supersonic range so that the sizeand weight of filter circuit parts is held to a minimum. The harmonicsare picked off the vibrator 18 by sixteen filters 19, of which only twoof the extreme limits of the scale are shown. The 16 filters feed,therefore, as will be shown, each bolometer and the ones shown feed thebolometers designated 11a and 11p. The upper represented circuit is forthe former and the lower one for the latter.

The currents from filters 19 are fed to an amplifier network 20 in whichit is amplified, and thence to transformers 21. The latter convert theamplified radio frequencies to low voltage, relatively large currentoutputs that lare impressed across special, balanced bolometer bridgecircuits '22. In the top bridge circuit, 11 indicates any bolometerstrip of which 11a s taken as an example and 13 is a bolometer made inan exactly identical manner but placed behind the shielding block 12 sothat it is shielded from radiation in the field of view. The resistors23 and 26 are of the ceramic fixed type. 24 is a variable resistor forbalancing the bridge. To reduce inductance and capacity to a minimum,resistor 24, shown in section in Fig. 7, consists of a thin metallicdeposit on the inside of the outer cylinder 32, variable pickof beingobtained by the spring fingers 33 of an inner concentric metal cylinder31 slidable therein. 25 is a resistor with the same dimensions andpickoff point as resistor 24 but with a thick instead of a thin metallicfilm. Its function is to balance that arm of the bridge in which itisinserted.

Resistors 24 and 25 are duplicates except that, as stated, 25 has athick layer of platinum on the inner side of cylinder 32 whereas 24 hasa thin one. By this means, when the resistance is varied by sliding theinner tube 31 longitudinally, the pickoff fingers 33 are moved togetherso that there will be the same change in the capacity or inductance, buta difference in the resistance. The tubes 31 of both resistors may bemade to be slidable by a single motion.

It is necessary to reduce all inductance and capacitance to very smallvalues, to make connections as short as possible and symmetrical, withsufficient rigidity to limit vibration. Each bridge is balanced for itsassociated frequency by means of a sensitive detector (not shown). Whenradiation falls upon a bolometer such as 11, the balance is upset and avoltage is impressed across pickofi: resistors 27 which are connected inseries.

Resistances 27 are connected to the bridge circuits 22 by leads 28 and29. All of them end at the resistances 27. The ends of the series ofresistors 27 are connected to an amplifier 30 which is provided with anautomatic volume control 34 (see Fig. 2) of the conventional type and isdesigned to act also as a limiter. The voltage impressed acrossamplifier 30 is equal to the sum of the voltage across the separateresistors 27. It is to be noted that the voltage across one resistor 27is not shunted by the other resistor 27 since each bridge is isolated bya transformer 21.

One function of resistors 27 is, therefore, to act tol,gether as a mixerfor the output of the sixteen bridge circuits. Amplifier 30 receives asignal from the output of said mixer, but the automatic volume control34 supplies suicient bias so that only the positive peaks of thestrongest signal come through the first section of the amplifier. Thesepeaks or pips have the same frequency as one of the bolometers, but areno longer sine waves. They have many Fourier harmonic components thatwould pass through a bolometer quadrant filter bank 40 wherein aredisposed the different output filters 35, 36, 37, 38, and also those ofa second series 61, 62, 63, and 64 in a frequency-selective steppedresistor bank 41 (Fig. 3). 62a, 63a and 64a are each four channelfilters connected respectively to the outputs of filters 35, 36, 37 and38. Filters 62a, 63a and 64a are each identical with 61, 62, 63 and 64,collectively. They would tend to actuate a number of control circuitsinstead of only one. To restore a sinusoidal wave shape, the pips arenow impressed on the control grid of a pentode tube 43, the plate ofwhich is connected in series with anti-resonant tank circuits 44, 45,46, 47 corresponding to the given bolometer frequencies. Only the tankcircuit with the resonant frequency of the pips is'excited, and ygives asine wave output which Awill pass through the proper output filter.

The Fourier harmonics of the pip excite some of the anti-resonantcircuits of the sine wave generator 39, but the Sine waves so generatedare smaller in amplitude than the sine wave produced by the fundamentalof the pip, and are readily suppressed by limiter 48 in Fig. 2.

Stepped attenuator lter circuit 41 comprises attenuators 65 to 68,inclusive, in series, respectively, with pickotf resistors 42a to 42d,inclusive, one pair such as 65 and 42a, for example, give a signal fourtimes as great as the signal from 68 and 42d, since resistors 65, 66, 67and 68 are to each other as the integers 1, 2, 3 and 4. It is thefunction of the limiter 48 (see Fig. 2) to reduce all signals to thesame voltage amplitude. This must be done without changing the Waveshape, for if, for example, the top of a wave were cut of, many newFourier component frequencies would be produced that would pass throughthe Wrong output filters. Therefore, since a multivibrator was used forthe input frequencies, they are to each other yas the integers chosen.Then if the top of the sine waves were to be cut olf so that itapproached the characteristics of a square wave, some of the chosen 16harmonics would be present in appreciable size, and each harmonic wouldbe routed through a different output filter. It is necessary thereforeto reduce not only the amplitudes of the various signal frequencies tothe same amplitude, but to carefully preserve the sinusoidal wave shape.

In the limiter 48, the given signal is impressed on `the grid of a tube49, the plate thereof being in series with a resistance 50, a battery51, and a tube 55 which is used as a variable resistance. The outputsignal is taken from across the tube 55 and will be nearly proportionalto the resistance of that tube if a large value of resistance 50 isused. The voltage drop across tube 55 is also impressed upon a resistor52, `a rectifier 53, and a battery 54. The grid pickot for tube 55 isderived from resistor 52. No current will ow through resistor 52 untilthe voltage drop across tube 55 is greater than the voltage of battery54. When this occurs, a relatively large voltage will be impressedacross the grid of tube 55 and its resistance will drop rapidly untilits IR drop just exceeds voltage from the battery 54.

Since the resistance of tube 55 is only a small part of the totalresistance in the plate circuit of the tube 49, there will not be anyappreciable change in the limiter output wave shape from the signalimpressed on the grid of tube 49. A condenser 56 is connected toresistance 52 by a sliding contact that may be adjusted so thatcondenser 56 is charged rapidly when rectifier 53 passes current, butrequires a number of cycles to discharge through the larger portion ofresistor 52. This action maintains the voltage across the grid of tube55 with only a gradual drop-off between positive peaks, and preventsappreciable distortion of the |wave shape. Preservation of the waveshape is aided by the fact that all frequencies reaching the limiterhave been made nearly the same size bythe automatic volume control andsine wave generator, the value of tank circuits 44, 45, 46 and 47 havingbeen adjusted for this result.

The output of the limiter 48 is divided by band lters 35, 36, 37 and 38,so that the signal frequencies from the bolometers of one quadrant arerouted via one of these filters to that quadrant control. If thestrongest signal is in the rst quadrant, the signal will go throughrectifier 69 (see Fig. 1) to the rst quadrant control 90. The size ofthe output signal will depend upon the bolometer strip from which itoriginated, being proportional to the distance of the bolometer stripfrom the center of the image area. 'I'he simple tuned circuits thatseparate the signals from the bolometer strips of a given quadrant aredesigned to have attenuation inversely proportional to the distance ofthe bolometer from the center. Thus if the signal from the bolometer 11awere the strongest, it would be the only one to reach the band filters,and would pass through composite lters 35 and one lter 61. The amplitudeof the signals reaching the lters 35 to 38, inclusive, is the samebecause of the action of the limiter 48, regardless of the size of thesignal or the bolometer strip from which it originated. But the topfilter 61 has such attenuation that a signal passed through the topfilter 61 produces about four times the signal that it would have if itcame through the bottom filter 64. The signal from bolometer 11a ispicked off from one of four stepped attenuators or pick-olf resistors42a, 42h, 42e and 42d in series and is impressed on 'rectifier 69, theoutput of which gives a voltage proportional to the angular displacementof the target from the center of the image field. The quadrant control90, which is a servomotor is therefore driven at a speed directlyproportional to the voltage of the rectifier output until it aligns thehorizontal axis of the missile with the source of the controlling heatsignal. When that has occurred, the `heat signal strikes the exactcenter of the bolometer bank where there are no sensitive surfaces.Therefore the servomotor stops for lack of signal-induced current untilthe missile wanders off its course sufliciently to start a newcorrection cycle.

superposition of diiferent waves in the mixing circuit (such as pickofrresistors 27) of Fig. 3 may lead for certain frequencies to seriousintermodulation so that the pips of Fig. 2d would be irregular inheight, and some might be depressed, giving an irregular pip frequency.It may be necessary on account of such conditions to eliminate themixing elements 27 and to apply an automatic volume control 70 in commonto the separate bolometer output circuits as indicated in Figs. 8 or 9.Thus in Fig. 8 one lead 29 from each of the sixteen bolometer bridgecircuits, is connected in common to the cathode biasing circuit 70, theother lead 28 being connected to the grids of the tubes 74. The pickotfresistors 72 are large relative to the plate resistors 73, so that theoutput from one pickof resistor 72 is not shorted too much by the otherresistors 72 in parallel through the plate resistors 73 to ground. Theone pip frequency that comes through the depressing bias common to thetubes 74 is impressed upon the grid of a tube 30, the output of whichfeeds into the sine wave generator 43 to 47, inclusive, of Fig. 3.

Or, as in Fig. 9, each bolometer bridge output circuit is impressedacross the grid of a tube of which only two are shown with theircircuits. The outputs of rectifers 81 and 81 charge a condenser 82 to avalue proportional to the largest bolometer output from the plates oftubes such as 80. The voltage of condenser 82 is impressed on resistor83, thereby furnishing a common bias and automatic volume control forall the tubes 80 with a time constant determined by the values ofcondenser 82 and resistor 83. If the largest signal is received frombolometer 11a only, the peaks of these signals will cause electronpulses to pass to the plate of the tube 80 and hence to a coil 84 inseries with battery 86 and resistor 88. This coil 84 is coupled to theantiresonant pickot circuit 85, which is tuned to the fundamentalFourier component of the signals impressed on tube 80. The pickoffcircuit 85 is excited and impresses a voltage across the limiter l48 ofFig. 2. Since the pickoff circuits 85 are similar to other circuits forother frequencies, only one being excited at any one time, the unexcitedones having a relatively low impedance, a single sinusoidal wave isproduced of the frequency of the bolometer with the largest signaloutput.

In use, the bolometer bank is mounted as shown in Fig. 5 at the focalspot of a fixed parabolic mirror 91, the lbolometer 13 facing the mirrorsurface. The mirror 91 with the bolometer bank is mounted in the nose(not shown) of the missile to be guided. The nose is made preferably ofa single piece of material transparent to heat rays such as silverchloride, without ribs so that there is no additional resistance to theheat signal than occurs in its passage through a sheet of uniformthickness. The mounting stem 92 may be hollow so that it can be utilizedas a conduit to bring electric wires 16 to the bolometer bank. Beforelaunching the missile, which may occur from ground by means of a ramp orcatapult or from an airplane in flight, the current sources 7 are turnedon, the openings in the bolometer chamber closed to keep out drafts andthe launching may then take place in the conventional manner.

Since a missile is customarily provided with explosives and a detonatingmechanism therefor, it is to 'oe understood that all such conventionaladditions may be made to the missile to promote effective destruction ofenemy units or installations.

Numerous modifications such as the substitution of equivalents willsuggest themselves to those skilled in the art and such modificationsmay be made Without departing from the spirit of the invention.

The invention claimed is:

l. In a heat seeker, the combination which comprises a bank ofbolometers disposed to receive heat signals and to represent an opticalfield of view in quadrants thereof, a Wheatstone bridge circuit for eachbolometer, said circuit including said bolometer in one of its arms,means for supplying oscillatory current to said bridge circuits, anoutlet channel for each circuit, said outlet channel governing anexternal control and a system of primary and secondary filters to routethe signal from a given bolometer to the proper output channel to directa control first by the primary filters to identify the originatingquadrant, then by secondary filters to identify that portion of theoriginating quadrant which has received the signal.

2. In a heat seeker, the combination which comprises a bank ofbolometers disposed to receive heat signals, a Wheatstone bridge circuitfor each bolometer, said circuit including said bolometer in one of itsarms, means for supplying oscillatory current to each of said circuits,a mixer for the output of all of said circuits, an amplifier for saidcombined signals, an automatic volume control for said amplifier wherebyto produce such a bias that only the peaks of the strongest signals aretransmitted, an electronic tube having a grid connected to receive saidpeaks, and also a plate, a system of tanks circuits connected to theplate of said tube whereby a sinusoidal Wave tuned to the same frequencyas that of the signal input peaks fed to the amplifier is created bysaid tank circuits and a system of filters connected to said tankcircuits for routing the sinusoidal waves to output channels indicatedby the frequency of the filters connected thereto.

3. In a heat seeker mounted upon a missile, a bank of heat sensitiveelements disposed as a bank to receive heat signals, an electricalcircuit capable of unbalance, said circuit including one of said heatsensitive elements exposed in a position where it can cause electricalunbalance when it receives a heat signal, means for electricallypowering said circuits with oscillatory current, means for creating adifferent current frequency in each circuit, a second bank of identicalheat sensitive elements which is shielded from heat radiation, eachindividual element thereof being included in some one of said circuitscapable of unbalance, with one of said exposed heat sensitive elementswhereby the shielded element will serve as an electrical standard ofreference, an amplifier connected to the output sides of said circuits,a system of filters connected to said amplifier to route the signals tothe proper channels for proportional control and stepped attenuatorsconnected to the outputs of said filters, said attenuators havingimpedance values which differ from each other so as to give signaloutputs which are proportional to the angular displacement of the targetwith respect to the center of the bank of exposed heatsensitiveelements.

4. In a heat seeker mounted upon a missile, the combination whichcomprises a bank of bolometers exposed to heat signals, an identicalbank of bolometers shielded from radiation, a heat absorbing blockbetween said banks, an electric circuit including a Wheatstone bridgefor each bolometer pair including a shielded and an exposed lbolometer,one arm of said bridge including a shielded bolometer and its companionarm including all exposed bolometer, the other two arms of said bridgeincluding respectively fixed resistors with negligible capacitance andinductance and a variable resistor having negligible inductance andcapacity in the one arm and a similar variable resistor in the other armso that inductance and capacity may be the same in both of the latterarms, an amplifier connected to the output of said bridge circuits and afilter system connected to the output of said amplifier and to themissile controls to route signals through appropriate channels to giveproportional control to the missile.

5. An automatic control circuit for mounting on a. missile said circuit,comprising in combination, the elements connected together in the ordernamed, a master oscillator, a multi-vibrator, filters for separating theoutput of the multi-vibrator into at least sixteen different harmonicchannel circuits, an amplifier for each circuit, a transformer for eachcircuit, a Wheatstone bridge for each circuit, said bridge having in itsfour arms respectively, a bolometer exposed to heat signals as a memberof a disc-like bank, a bolometer shielded from heat signals, fixed andvariable resistance in which the inductance and capacity are negligible,fixed and variable resistance substantially devoid of inductance andcapacity exactly balancing the just mentioned fixed and variableresistance, a mixer for the bridge outputs, an amplifier for said mixeroutput, an automatic volume control for said amplifier output whereby totransmit only the peaks of the strongest signals, a sine wave generatorto receive said peaks and to incorporate them into a sine wave, alimiter to bring the sine wave to constant voltage amplitude, fourfilters for the output of said sine wave generator to route the signalscurrent therefrom according to the quadrant of the disc of bolometersfrom which their location and frequency originates, four tuned filtersfor each one of said quadrant-locating filters, attenuators in serieswith each filter, said attenuators together constituting a series havingthe relation and value of integers in respect to each other, a rectifierfor the output of said attenuators, said rectifier being connected toone control of said missile whereby said control will be actuableproportionally in at least four steps according to the irnpedance valueof the attenuators through which the actuating current s transmitted.

6. In an automatic control circuit according to claim 5, themodification which comprises a triode electronic tube for each bridgeoutput circuit, said circuit being connected to the grid of said tube,an automatic volume control furnishing a common bias for each of saidtubes whereby only the peaks of the signals received from the bridgecircuit will pass to the plate of said tube, an antiresonant pickotfcircuit tuned to the fundamental Fourier component of the peakstransmitted to the plate of said tube, a limiter connected to the outputof said pickoff circuit, and a limiter to bring the sine wave generatedby the tube to constant voltage and amplitude.

7. In a heat seeker, a proportional control system comprising aplurality of bolometer-receivers an amplifier, an automatic volumecontrol therefor arranged to produce such a bias that only the peaks ofthe strongest signals from the bolometer-receivers will come through,electronic elements including a vacuum tube of at least three electrodesincluding a grid and a plate, the grid being adapted to receive saidpeaks, a system of tank circuits connected to the plate of said tube sothat a sinusoidal wave may be picked off that tank circuit which istuned to the same frequency as the signal input peaks.

8. In a heat seeker, a proportional control system comprising incombination a group of bolometer-receivers in which the units arecharacterized by individual impressed frequencies, an amplifier, anautomatic volume control on said amplifier, said control lbeing arrangedto pass only the peaks of the largest signals, a plurality of tankcircuits arranged to generate a sinusoidal wave tuned t0 the frqueny Ofthe incoming signal affecting one bolometer-receiver and filter circuitsarranged to route said sinusoidal Wave to the proper mechanical controlelement and to exert proportional control thereon.

9. In a heat seeker, a proportional control according to claim 8 havingin addition an attenuator in each of the lter outputs, said attenuatorsbeing so arranged as to give signals proportional to the angulardisplacement of the target with respect to the axis of the seeker.

10. In a heat seeker, a proportional control system comprising areceiver system in which dierent units are characterized by differentfrequencies of impressed voltage, a bridge in which a receiver unitexposed to radia tion forms one arm of the bridge and a substantiallyidentical unit shielded from radiation forms the other symmetrical arms,the remaining arms comprising xed resistors having negligiblecapacitance and inductance in series, in one arm, with a doubletelescopic cylindrical resistor formed by a thin film metal deposit uponone cylinder and an outer metallic cylinder with spring ngers to slideover it and a similar cylindrical resistor in the remaining arm of thebridge, the outer cylinder having the same position but there being athicker metallic deposit on the inner cylinder so that the inductanceand capacitance may be the same in both arms, an amplifier, and a liltersystem arranged to route the signals from the bridge through the properchannels for proportional control.

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